Add signal to the network parser.

Some tests like network_WiFi_VerifyAttenuator may take
advantage of knowing the singal strength before connecting
to a network.

BUG=None
TEST=network_WiFi_VerifyAttenuator, unittest

Change-Id: Ib0aa13e271ca8afee1c839b044efd3923f14c8b2
Reviewed-on: https://chromium-review.googlesource.com/275061
Trybot-Ready: Kris Rambish <krisr@chromium.org>
Tested-by: Kris Rambish <krisr@chromium.org>
Reviewed-by: Christopher Wiley <wiley@chromium.org>
Commit-Queue: Kris Rambish <krisr@chromium.org>
2 files changed